In OEM bluetooth hands-free car kit applications, a bluetooth transceiver/voice processor is usually pre-wired to car stereo speakers. However, in aftermarket applications, either the car electronic system has to be modified or an additional speaker has to be placed in the car. Both these methods are cumbersome as they generally have to be performed by a qualified car-electronic technician. To avoid this problem, some bluetooth hands-free car kit manufacturers use a short range FM transmitter to broadcast voice and music from the bluetooth hands-free car kit to the car stereo. This solution uses one of many off-the-shelf FM transmitters.
Such transmitters are based on analog technology where the carrier frequency is changed by changing a capacitance of an LC oscillator (L-inductor, C-capacitor). The capacitance of the LC oscillator is changed by changing the bias voltage of a varactor diode with a trimmer potentiometer. Because the frequency of LC based oscillators is very sensitive to temperature and aging and because the temperature in the car can vary by as much as 50° C. (the car left on the sun vs. car with air-conditioning on), the carrier frequency of these FM transmitters can drift by as much as 10%. Hence, FM transmitters require very frequent retuning, making them not very useful for in-car applications. These solutions also require a voice/music signal in analog domain or a Digital-to-Analog (D/A) Converter to convert a voice/music signal from the digital to analog domain in systems where voice/music is stored, processed and transmitted digitally.